Test for determining corrosion resistance of stainless steel



United States Patent Ofi ice 3,236,599 Patented Feb. 22, 1966 This invention relates to a test which makes possible the rapid determination of the corrosion resistance of chromium-bearing stainless steels.

Stainless steel, particularly chromium ferritic steel, is widely used in the automotive industry, for example, in the form of exterior fittings, such as wheel covers, window frames and other decorative trim. These exterior fittings are subject to corrosive attack, particularly in northern climates during the winter months due to the water soluble salts which are often applied to road surfaces to mitigate the effects of ice and snow. As a result of these salts, road sludge contains such corrosive elements as sulfates, sulfites, thiosulfates, chlorides, alkaline earths and alkalies. These salts when splashed upon stainless steel causes pitting of the stainless steel.

Accelerated tests for determining the corrosion resistance of a given stainless steel have been devised and they are a great improvement over time consuming actual road trials. However, such tests still require considerable time, sometimes up to 16 hours, as well as complicated equipment including accurate timing and cycling devices.

Accordingly, it is the object of the present invention to provide a rapid, dependable test procedure upon which the corrosion resistance of the stainless steel, when sub jected to actual road conditions, may be predicted.

Broadly stated, the test of the present invention comprises a thermographic procedure which will rapidly generate a corrosive attack on a stainless steel test specimen and visually display the severity of the same, so that the relative corrosion resistance of the stainless steel may be readily determined.

More specifically, the thermographic test procedure of the invention comprises subjecting at an elevated temperature and high humidity, a polished and degreased section of the particular stainless steel to be tested to the action of a solution of corrosive salts which are contained in a gelatin or other water dispersible colloidal gel coated absorptive member. The corrosive salts will generate a corrosive attack on the test specimen. The iron salts and chromium salts which are formed as a result of such attack will remain embedded in the gelatin when the gelatin coated member and the test specimen are separated, and the extent and quantity of embedded salts is a visually observable measure of the relative corrosion resistance of the test specimen. For best results, the embedded iron and chromium salts are chemically treated to convert them to salts which posses a highly distinct or brillant color so that the visual observation of the same is simplified.

In the practice of the invention, the polished section of the surface to be tested may be thoroughly degreased by any suitable means such as washing with solvents, for example, acetone, perchloroethylene, and carbon tetrachloride. Suitable gelatin coated members for use in the practice of the invention include dye transfer paper and the type printing paper used in the photographic industry from which the silver halides have been removed by treatment in a hyposulfite fixing solution. Corrosive salt solutions which are suitable in the practice of the invention include chloride-distilled water solutions having a concentration range of chloride of from 0.5 to 35 percent, the chloride ions being furnished suitably by ammonium chloride, an alkali chloride or alkaline earth chlorides. The pH of the salt solution should be adjusted in order that optimum ionization of the salt is achieved.

The preferred corrosive salt solution for use in practicing the test procedure of the invention may be prepared as follows:

Dissolve in 525 ml. of distilled water (having a pH of not less than 5.5) the following salts in the order and amounts given.

(1) Sodiumsulfate (Na SO anhydrous reagent grade 0.500 g. :2 mg. (2) Sodium sulfite '(Na SO anhydrous reagent grade 0.250 g. :2 mg. (3) Sodium thiosulfate (Na S O anhydrous reagent grade 0.100 g. :1 mg. (4) Sodium chloride (NaCl), C.P.

grade 52.5 g. :1 g.

Dissolve in a second 525 ml. of distilled water (having a pH of not less than 5.5) 52.5 g.i0.l g. of pure calcium chloride dihydrate, CaCl .2H O.

When both solutions are completely clear, very slowly add the second solution (of CaCl to the first solution with continuous stirring. If properly done, the final solution will be clear; if mixing is done too rapidly, a precipitate may form in which case the solution should be filtered. The pH of the clear solution should be immediately adjusted to 9.3 :005 using dilute aqueuos solutions of sodium or hydrochloric acid.

The actual test procedure of the invention comprises intimately contacting the gelatin coated surface of the absorptive member which has been saturated with a corrosive salt solution with a clean and degreased section of the stainless steel specimen to be tested, and subjecting the stainless steel to elevated temperatures at high humidity. Temperatures between about 60 C. and 115 C. and a humidity in the order of have been found to be most suitable for the practice of the test of the invention. The heating may suitably be carried out by placing the test specimen and the absorptive member in an inert vapor impervious enclosure, which is then placed in an oven. During this heating period, if the steel is corrosively attacked, iron and chromium salts will form. When the gelatin coated paper is subsequently separated from the test specimen, any iron and chromium salts which were formed will be carried away in the gelatin. The location and size of these salt deposits will correspond exactly to the areas of the stainless steel sample which were attacked and are proportional in number and area to the severity of the attack. The severity of the attack may be graphically illustrated by treating the iron and chromium salts with a potassium ferricyanide solution to give the salts a bluish color.

A specific example of the test of the invention is the following.

Example I A 3 inch by 4 inch by 0.02 inch thick AISI No. 430 chromium stainless steel test specimen, a surface portion of which has been degreased with a solvent of acetone, was centered on a piece of treated photographic print paper which had been saturated with the specific corrosive salt solution detailed above by soaking the paper in the solution for about 10 minutes. The photographic print paper was of such a size that a /s inch border of uncovered steel was left around the edges. The facing surface of the paper contains an approximate 0.002 inch layer of gelatin. Intimate contact between the gelatin coated paper and the test specimen was insured by passing a roller over the two,

The specimen and the gelatin coated paper were then placed in a polyethylene bag with a minimum of entrapped air. The open end, of thebag.wasflfoldedovern It should be further appreciated that whilethe duraseveral times and sealed in order to insure a high humidity atmosphere. The bag was then placed in an oven whichwas at a temperature of 100 C.- for about minutes. At the end of the heating period the bag was quickly opened and the gelatin coated paper was stripped from the metal before the latter cooled'appreciably, in order to prevent sticking. The gelatin surface paper was immediately dipped into a 1 percent aqueous solution of potassium ferricyanide and held there until no further increase in blue color of the deposited iron and chromium salts was noted (approximately seconds to 1 minute). Since there was almost no lateral difiusion of the iron salts transferred to the gelatin, the blue color thus developed, clearly delineated the area corresponding to those on the steel which were attacked by the corrosive solution. The excess potassium ferricyanide solution was washed out in running water and the paper was dried. The paper may be kept as a permanent record, if desired.

The success of the test of the invention was determined by comparing the indicated degree to resistance or attack as measured by the test of the invention with the same as measured by the commonly practiced dip and dry test which consists of alternately dipping the specimen :in the above outlined salt solution followed by drying at a temperature of, 100 F. to 120 F. The dipping and drying is continued for' about 8 hours which is approximately 288 complete cycles.

These two tests were applied to a series of chromium steels with the following results.

Steel, AISI No. Inventive Test Dip and Dry Test Very re i tant Moderate attack Heavily attaeked Very resistant. Moderate attack. Heavily attacked. Moderate attaek Moderate attack.

1 Moderate attack Moderate attack.

Heavily attacked Heavily attacked. Moderate attack Moderate attack.

tion of the test (and accordingly, the severity of attack) may be varied somewhat, it should be held constant when comparing the relative corrosion resistance of a series of specimens.

We claim:

1. In a test for predicting the corrosion resistance of a stainless steel when subjected to actual road conditions comprising steps of preparing a polished and degreased surface section of a specimen of said stainless steel, uniformly saturating an absorptive member having a gel coating on at least one surface with a corrosive salt solution, intimately contacting the polished and degreased surface of said specimen with the gel coated surface of said solution saturated member, subjecting said specimen and said member to an elevated temperature and a high humidity for a time sufiicient to generate corrosive attack on said specimen, and chemically treating any iron and chromium salts formed and imbedded in the gel as a result of the corrosive attack to convert them to salts having a highly distinct and visible color; the improvement wherein the temperature is at least C. and the humidity is of the order of about percent.

'2. The test procedure of claim 1 wherein said treatment at an elevated temperature is accomplished by placing said specimen and said member in an inert vapor impervious enclosure with a minimum of entrapped air and placing said enclosure in an oven which is at a temperature of approximately 100 C. for about 10 minutes.

Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 2, p. 523 (1922), Longrnans, Green and Co., New York.

Uhlig: Corrosion Handbook (1948), pp. -173, 1030-34, and 1077-83.

MORRIS O. WOLK, Primary Examiner. JAMES H. TAYMAN, JR., Examiner. 

1. IN A TEST FOR PREDICTING THE CORROSION RESISTANCE OF A STAINLESS STEEL WHEN SUBJECTED TO ACTUAL ROAD CONDITIONS COMPRISING STEPS OF PREPARING A POLISHED AND DEGREASED SURFACE SECTION OF A SPECIMEN OF SAID STAINLESS STEEL, UNIFORMLY SATURATING AN ABSORPTIVE MEMBER HAVING A GEL COATING ON AT LEAST ONE SURFACE WITH A CORROSIVE SALT SOLUTION, INTIMATELY CONTACTING THE POLISHED AND DEGREASED SURFACE OF SAID SPECIMEN WITH THE GEL COATED SURFACE OF SAID SOLUTION SATURATED MEMBER, SUBJECTING SAID SPECIMEN AND SAID MEMBER TO AN ELEVATED TEMPERATURE AND A HIGH HUMIDITY FOR A TIME SUFFICIENT TO GENERATE CORROSIVE ATTACK ON SAID SPECIMEN, AND CHEMICALLY TREATNG ANY IRON AND CHROMIUM SALTS FORMED AND IMBEDDED IN THE GEL AS A RESULT OF THE CORROSIVE ATTACK TO CONVERT THEM TO SALTS HAVING A HIGHLY DISTINCT AND VISIBLE COLOR; THE IMPROVEMENT WHEREIN THE TEMPERATURE IS AT LEAST 60*C. AND THE HUMIDITY IS OF THE ORDER OF ABOUT 100 PERCENT. 